1. Field of the Invention
The present invention relates to a liquid crystal display, particularly to a bend alignment mode liquid crystal display.
2. Description of the Related Art
Liquid crystal displays (LCDs) are advantageous in achieving lower-profile, lighter weights, lower power consumptions, etc. as compared with cathode ray tubes (CRTs). The liquid crystal displays have a liquid crystal cell and a pair of polarizing plates placed at both sides of the cell. The liquid crystal cell contains liquid crystal molecules, two substrates for enclosing the molecules, and an electrode layer for applying voltage to the molecules.
An alignment layer is generally formed on each of the substrates to align the enclosed liquid crystal molecules.
Optical compensatory films (phase difference plates) are often formed between the liquid crystal cell and the polarizing plates to remove undesired coloration of image colors emitted by the liquid crystal cell.
A multilayered structure composed of the polarizing plates (polarizing films) and the optical compensatory films acts as a polarizing plate. The optical compensatory film can have a function of enlarging a viewing angles of the liquid crystal cell. Stretched retardation films have been used as the optical compensatory film.
Also using an optical compensatory film having an optically anisotropic layer containing a discotic compound, which can be used instead of a stretched retardation film, have been proposed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 06-214116, U.S. Pat. Nos. 5,583,679 and 5,646,703, and German Patent No. 3,911,620 A1.
The optically anisotropic layer is formed by aligning the discotic compound and by fixing the alignment state. The discotic compound generally has a large birefringent index. Further, the discotic compound can be in various alignment states.
Thus, the discotic compound can be used for producing an optical compensatory film with non-conventional optical properties, which such stretched retardation films cannot have.
Liquid crystal displays using a bend alignment mode liquid crystal cell, wherein an alignment direction of rod-shaped liquid crystal molecules by the one cell substrate of the liquid crystal cells is substantially opposite, or symmetry, to that of the molecules by the other cell substrate, are proposed in, for example, U.S. Pat. Nos. 4,583,825 and 5,410,422.
In such liquid crystal displays, wherein the rod-shaped liquid crystal molecules by the one cell substrate of the cell substrates are oppositely aligned to the molecules by the other liquid crystal cell, bend alignment mode liquid crystal cells of the liquid crystal displays thus have a self optical compensatory function. Whereby, the liquid crystal mode is referred to also as an OCB (Optically Compensatory Bend) liquid crystal mode.
And the bend alignment mode liquid crystal displays are advantageous in high response speeds.
Thus, the bend alignment mode is characterized in wider viewing angles and faster response speed as compared with common liquid crystal modes such as TN and STN modes.
However, in comparison to CRTs, the bend alignment mode displays are still to be improved. It seems possible that the bend alignment mode liquid crystal displays can be further improved using an optical compensatory film as in the case of common liquid crystal mode-displays.
However, the optical compensatory films composed of the conventional stretched retardation films show insufficient optical compensatory function when they are adopted for the bend alignment mode liquid crystal displays.
Therefore, as described above, the use of the optical compensatory films having a transparent substrate and an optically anisotropic layer containing a discotic compound instead of the stretched retardation films has been proposed.
Also bend alignment mode liquid crystal displays using an optical compensatory film containing a discotic compound have been proposed in JP-A No. 09-197397, International Publication No. WO96/37804, etc.
By using the optical compensatory sheet containing a discotic compound, viewing angles of the bend alignment mode liquid crystal displays is remarkably improved.
It has been pointed out in JP-A 11-316378, etc. that, when the optical compensatory film containing a discotic compound is used in the bend alignment mode liquid crystal displays, light having a particular wavelength leaks and cause undesired coloration of images.
JP-A 11-316378 reported that the coloration is caused by wavelength dependence of transmittance of an ellipsoidal polarizing plate (a multilayered structure composed of polarizing films and optical compensatory films).
Further, it is reported that a maximum optical compensatory effect for a bend alignment mode liquid crystal cell can be achieved by arranging the optically anisotropic layer and the polarizing film such that an average angle of orthographic projections of normal lines of a discotic plane of the discotic compound to the in-plane transmission axis of the optically anisotropic layer becomes substantially 45 degrees.
Various proposals have been made on reduction of undesired coloration and prevention of tone reversal in the bend alignment mode liquid crystal displays using the optical compensatory film containing a discotic compound in Japanese Patent Application (JP-B) No. 3056997, JP-A 2002-40429, etc.
Meanwhile, the above-stated conventional bend alignment liquid crystal displays apply a relatively high-applied voltage when displaying white, thus they achieve low contrast which is the difference between white brightness and black brightness. However, bend alignment liquid crystal displays have been improved to achieve higher contrast by lowering the applied voltage when displaying white. Therefore, viewing angle compensatory films adapted for such improved bend alignment liquid crystal displays should be used to improve both contrast and viewing angles of such bend alignment liquid crystal displays.